A plano-stereoscopic display is one which produces the depth sense, stereopsis, by presenting appropriate left and right planar images to each respective eye. For the observer to be able to fuse these two planar images into a single stereoscopic view, the image for one eye must be isolated from the other. If the left eye, for example, also sees all or a portion of the intensity of the right image, there will be a perceived doubling of the image or "ghosting." Incomplete left and right channel isolation, or crosstalk, is of great concern to the designer of a stereoscopic system.
In a plano-stereoscopic field-sequential display, two factors may contribute to crosstalk: (1) phosphor decay, or afterglow, and (2) dynamic range of the shutters. The choice of cathode ray tube (CRT) phosphors in video systems is based on various display needs. In field-sequential stereoscopic technologies, use of short-persistence phosphors is preferred. Fortunately, many video and computer graphics monitors use display tubes with medium short phosphors which have a decay time which is brief enough to produce acceptably low levels of ghosting. If the phosphors' afterglow persists into the adjacent field to a significant extent, there will be crosstalk and observed ghosting because the right eye, for example, will see a reduced-intensity image of the unwanted left field.
Shutter performance is also of great concern, and two important parameters of shutter performance are the transmission of the device in its open state and its dynamic range. Dynamic range is defined as the ratio of the transmission of the shutter in its opened state to the transmission of the shutter in its closed state. It should be appreciated that the contribution to crosstalk made by the incomplete occlusion of the shutter manifests itself to the eye while the shutter is closed, while the contribution of crosstalk produced by phosphor afterglow is made during the open portion of the shutter's cycle.
In order to understand the invention it is helpful initially to review conventional shutter design, such as that of FIG. 1. The FIG. 1 shutter includes first linear polarizer PA1, surface mode liquid crystal cell LC, and second linear polarizer PA2. The absorption axis of first polarizer
is parallel with the horizontal, and the absorption axis of second polarizer PA2 is parallel to the vertical. The axes of the first and second linear polarizers are bisected by the alignment axis LCA of liquid crystal cell LC. The liquid crystal cell's axis is at 45 degrees to the vertical. The liquid crystal device shown in FIG. 1 is the type called a surface mode cell whose construction is taught by Fergason in U.S. Pat. No. 4,385,806, issued May 31, 1983.
Liquid crystal cell LC is made up of a layer of liquid crystal material sandwiched between two flat and parallel glass sheets, coated with substrates on their inside facing surfaces. These substrates are thin, transparent, electrically conductive layers such as indium tin oxide. It is through this layer that the electric field is applied to the liquid crystal. Another thin coating called an alignment layer is deposited on top of the conductive layer. It is the function of this alignment layer to impose a preferred orientation on the liquid crystal molecules. Such an orientation is necessary for the shutter to exhibit the desired electro-optic effect. One way in which this orientation effect can be accomplished is to rub the alignment layer with a special material. The rubbing direction on one substrate is parallel (or anti-parallel) to the rubbing direction on the other substrate, as taught by Fergason in U.S. Pat. No. 4,385,806.
The surface mode cell described above is a capacitor and can be charged to a high or low electric potential. A surface mode cell attains its speed because only a thin layer of liquid crystal molecules near the substrate actually moves as the electric potential is switched.
In the arrangement described above, the liquid crystal material is in a retardation state when at a low electrical potential, and is in an isotropic state when at a high electrical potential. In the low potential state, the molecules near the surface maintain the alignment imposed on them by rubbing of the director alignment layer, and when in the high potential state, the molecules become aligned parallel to the electric field and are therefore isotropic rather than anisotropic. A typical high potential state is between 25 to 50 volts peak to peak, and a typical low potential state is between 0 to 10 volts. By using different voltage settings for the low potential, one can tune the retardation of the liquid crystal cell. Generally speaking, the high voltage state determines how quickly and completely the cell will "turn on," and the low potential state determines the value of retardation. By adjusting the voltages it is possible to vary the dynamic range of the shutter continuously.
The prior art cell described above will produce a dynamic range of approximately 15:1, even with a very high extinction polarizer, such as an HN22 linear polarizer manufactured by Polaroid. That is, the ratio of the intensity of light transmitted to the light occluded will be 15:1. For many applications, especially for use in a stereoscopic shutter, this is an inadequate dynamic range. In point of fact, a stereoscopic shutter requires a dynamic range of many times this ratio.
One way to improve performance that has been attempted is to place two conventional shutters in optical series as shown in FIG. 2. In FIG. 2 we see two liquid crystal shutters in optical series, shutter I (comprising first linear polarizer 100, LC cell 101, and second linear polarizer 102) and shutter II (comprising second polarizer 102, LC cell 103, and third linear polarizer 104) sharing second polarizer 102. The absorption axes
and PA3 of polarizers 100 and 104 are parallel and both are orthogonal to the absorption axis PA2 of polarizer 102. Liquid crystal cells 101 and 103 have their rub axes parallel and at 45 degrees to the vertical. They are driven electrically in phase.
By placing two shutters in optical series (as described in the preceding paragraph) it is possible to greatly improve the dynamic range, but this expedient inherently increases the cost, weight, thickness, and complexity of the shutter. This expedient also increases the system's power requirements, and he use of three polarizers decreases the system's light transmission. In many applications, for example, for stereoscopic visors or aerospace applications, power, transmission, and weight characteristics are of critical importance.
Because of the defects inherent in conventional shutter systems, we undertook a research effort to develop a liquid crystal shutter which would have high dynamic range, good transmission, and high speed. Good transmission is important because image brightness is a desirable quality in a display system. In addition, the rise or decay time (transitions from open to closed or closed to open) of the shutter must take place within the vertical blanking interval of the video signal, which generally is about a millisecond. If the shutter's rise or decay is slower than the blanking interval, a portion of the image may be occluded.
In addition to stereoscopic video image selection applications, there are many other applications for a high dynamic range shutter of the inventive type. These applications include use of the high dynamic range shutters as replacements for mechanical shutters used in motion picture cameras and projectors, or as replacements for mechanical focal plane or between-the-lens shutters used in still photography. Moreover, the ability to vary shutter density continuously and incrementally with voltage makes a high dynamic range shutter system of the inventive type useful as a variably density filter in photographic or videographic camera applications. Such a variable neutral density filter could be used in place of, or in addition to, a lens' iris diaphragm for exposure control.
However, until the present invention, it was not known how to design or construct a high dynamic range, variable density liquid crystal shutter of the type having none of the above-described disadvantages of conventional shutter systems.